Eviolite Pumkaboo can have higher stats, but Gourgeist is still better. Though Eviolite Pumkaboo-Super has a maximum defense base and special defense base of 393 and 343 compared to Gourgeist's slightly lower 377 and 273 respectively with 252 EVs, 31 IVs and a positive nature, Gourgeist still has a higher HP total. Not only that, it can also hold an item, which can help more as Pumkaboo is always forced to hold Eviolite.
When talking about bulkiness, here are calcs:
252+ Atk Abomasnow Ice Shard vs. 252 HP / 252+ Def Eviolite Pumpkaboo-Super: 68-84 (21.1 - 26%) -- 1.5% chance to 4HKO
252+ Atk Abomasnow Ice Shard vs. 252 HP / 252+ Def Gourgeist-Super: 72-86 (19.2 - 22.9%) -- possible 5HKO
0- SpA Abomasnow Blizzard vs. 252 HP / 252+ SpD Eviolite Pumpkaboo-Super: 138-164 (42.8 - 50.9%) -- 3.1% chance to 2HKO
0- SpA Abomasnow Blizzard vs. 252 HP / 252+ SpD Gourgeist-Super: 174-206 (46.5 - 55%) -- 67.6% chance to 2HKO
There isn't a large difference between the damage, but Gourgeist ends up more physically bulky due to almost same defense but a higher HP. Pumkaboo has a bit of a more SpD, but ultimately, Gourgeist is better as it has the freedom to hold an item, unlike Pumkaboo which is forced to have Eviolite.
As you haven't clearly specified the formats you're playing with, I'm just gonna generally put it as Gourgeist is just better every tier both are allowed, but for example if you're playing LC then Gourgeist is banned for which you can choose Pumpkaboo. Gourgeist also has more offense stats, so it can also be a wallbreaker with Choice Band set (though you have asked to compare their bulk not offenses).